1. Reference to Related Applications
This invention relates to U.S. Application Ser. Nos. 475,419 and 475,418, now U.S. Pat. No. 4,484,026,filed Mar. 15, 1983 and Mar. 15, 1983 and having a common ownership. The disclosure of such applications are incorporated herein by reference.
2. Field of the Invention
This invention pertains to a digitizer pad or touch tablet usable as a discriminating electrical contact sensor or a graphical input terminal. Two or more, generally parallel, planar and spaced sheets having conductive or resistive coatings thereon, are pressed together by a stylus force or an operator's finger force at essentially one contact point at any position on the top sheet surface to electrically indicate the x-y coordinates of the point touched. More particularly the invention is directed to a means for normally separating the parallel sheets and returning the depressed sheet into parallelism with the other sheet when the stylus or human operator finger force is removed. "Digitizer Pad" as used herein means a pad or touch tablet having at least 10 electrical input points per square inch on its facing operable surfaces, including an infinity of points as in a continuous resistive surface.
3. Description of the Prior Art
Digitizer pads are illustrated in U.S. Pat. No. 3,911,215 wherein top and bottom sheets having conductive or resistive surfaces thereon are separated by a gridwork of particularly sized and spaced multiple projections on a surface of one of the sheets in facing contact with a surface of the other of the sheets. A stylus having a tip of prescribed radius of curvature is used to depress a flexible first sheet between adjacent projections so that contact is made between facing conductive or resistive layers by the projections. In this device the peripheral edges of the sensor sheets are held in a device frame separated by peripheral projections. U.S. Pat. No. 3,969,585 shows a graphical input terminal in which top and bottom resistive sheets (carbon on plastic) are separated by an air gap with the top sheet tensioned over a frame by Neoprene tubing and the sheets spaced from one another by a 1.5 mm plastic spacer. Each of these designs necessitate very close spacing i.e. 0.129-1.5 mm, of the facing surfaces of the sheets which results in a tablet or pad of unwanted extreme sensitivity which can emit spurious signals when brushed by a operator in case of the '585 patent or require a particular stylus and stylus force in the case of the '215 patent. Each of these prior art devices relies on the inherent flexibility of the top sheet itself to effect return to the separated or spaced position. This tends to limit the selection of materials and thickness of the top sheet. Further, with respect to each of the prior art patents there is little "feel" of any positive returning action especially when ambient temperatures are high and the top plastic sheet softens and sags. Thus, there has been a need of a digitizer pad having an uninterrupted output, a self-supporting top surface and a simple assembly method.